It is known to take various measures to reduce the IR and RCS signatures of ‘stealth’ aircraft such as UCAVs (Unmanned Combat Air Vehicles). The engine exhaust of such aircraft is a significant contributor to the IR signature, and it is known to take measures to reduce the temperatures of both the exhaust gases issuing from the exhaust nozzle and of the aircraft components surrounding the exhaust nozzle. This has been achieved in the past by constructing the exhaust nozzle as a twin-walled structure, so that cooling air can flow between the walls, to emerge into the exhaust gas flow through effusion cooling holes in the inner wall. The inner wall has been constructed as a liner made up of a plurality of tiles supported from the outer wall or nozzle casing of the engine.
In a previous proposal, the nozzle casing of an exhaust nozzle for a UCAV has a generally trapezoidal flow cross-section defined by top and bottom walls and a pair of side walls which interconnect the top and bottom walls. The side walls are relatively short in the flow direction, and the top and bottom walls have V-shaped profiles projecting beyond the side walls in the downstream direction (with respect to gas the direction of flow through the nozzle).
The liner in the previous proposal is at generally the same distance from the nozzle casing around the circumference of the nozzle. Consequently, as seen in cross-section, the flow passage for the cooling air has a constant width around the exhaust nozzle.
The nozzle casing needs to be very stiff, particularly at the nozzle exit, in order to maintain its alignment with the airframe in which it is installed, and to avoid excessive loads on the liner. The nozzle casing needs to be sufficiently stiff to resist pressure loads which tend to deform it outwardly, to assume an oval cross-section. Furthermore, the cooling air pressure tends to deflect the overhanging downstream end portions of the top and bottom wall portions in the direction away from the exhaust centreline, while the same cooling air pressure tends to deform the liner in the direction towards the exhaust centreline. The result of these effects is to widen the gap between the nozzle body and the liner, particularly in the downstream end regions, and this can increase the RCS signature and can also allow the uncontrolled escape of cooling air from the cooling passage. Distortion of the nozzle casing upsets the aerodynamics of the exhaust nozzle, affecting the distribution of cooling air over the liner and into the exhaust gas flow.
In order to achieve adequate stiffness in the nozzle casing of the previous proposal, the nozzle casing has a substantial thickness, and is consequently heavy.